A controller for detecting the end of an operation (often called the "endpoint") on a material and particularly on semiconductor wafers will typically detect the endpoint by detecting a change in light being reflected or transmitted from the material. In one system for doing this an optical emitter such as a light-emitting diode (LED) produces light which strikes a wafer surface and is reflected back to an optical detector. In another system natural light produced by a reaction process is monitored by a detector and the rate of change of this natural light in certain frequency bands is used to detect the endpoint of an operation. In both cases the detected light intensity is a measure of the state of the material being processed. The state of the material being processed may be measured by the material's reflectivity, by the chemical constituents of the material, or by the index of refraction of the material. A change in reflectivity indicates the process endpoint for metal etching, while the end of thin-film-interference oscillation signals may signal the endpoint for dielectric etching and photoresist development.
During development and removal of photoresist or during dielectric etching, interference fringes are a direct indication of resist dissolution or dielectric removed. The breakthrough to the underlying substrate (which might be a wafer surface, for example) that occurs when the photoresist or dielectric is removed is referred to as the endpoint and, as shown in FIG. 1, is recognizable as the point where the interference signal becomes nearly flat. The total process time consists of the time (A) which is required to reach breakthrough (endpoint), and any additional time (B) needed to clear out the resist or the dielectric completely. The time (B) is generally referred to as the overdevelopment or overetch period and will depend upon the nature of the material being removed.
The total process time is equal to the sum of the time to endpoint plus the overdevelopment or overetch time. For simplicity, the phrase "overprocess time" will be used in this specification to mean either the overdevelopment time or the overetch time depending on whether a photoresist is being developed and thus removed or a layer of material is being removed.
The actual signals observed can, and in many cases will, vary drastically from an ideal interferogram pattern. Variations in reflectivity from the substrate layers (such as Si, Poly-Si, Al, SiO.sub.2 and Si.sub.3 N.sub.4) topography, substrate roughness, as well as process variables, will affect the strength and characteristics of the reflected signal.
Establishing precise endpoint time is essential in determining the start of the overprocess period, so that the total process time is tightly controlled. A reliable and accurate process control system must be able to recognize the endpoint under any variable signal conditions. For instance, in semiconductor processing where endpoint detection is important, wafer conditions and the wafer-to-sensor distance may vary. In order for the endpoint to be accurately detected, small changes in the reflected light intensity must be measured and changes in light intensity due to effects other than the process being monitored must be eliminated.
Endpoint detection sensors capable of meeting these stringent requirements can suffer from several problems. Ambient light can interfere with the detection of light reflected from the wafer by causing noise in the monitored signal. As the wafer spins during processing, the wafer may wobble or tilt, causing the reflected light to vary. This artifact may interfere with endpoint detection.